Xylene isomers are produced in large quantities from petroleum and serve as feedstocks for a number of important industrial chemicals. Paraxylene is the principal feedstock for polyester. Orthoxylene is used to produce phthalic anhydride. Metaxylene is used for products such as plasticizers, azo dyes, and wood preservatives. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is a less desired component of C8 aromatics.
Among the aromatic hydrocarbons, xylenes and benzene are of substantial importance. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Most commonly, toluene, C9 aromatics, and C10 aromatics are reacted to yield benzene and C8 aromatics from which xylene isomers are recovered. Processes for doing this go by the names of toluene disproportionation, selective toluene disproportionation, and transalkylation. While the feedstock for these processes may vary depending on availability and economics, they all have the same goal of maximizing the production of xylene isomers typically through methyl transfer reactions. In some cases these processes also include reactions where molecules like methylethylbenzene are converted to ethane and toluene and this toluene in turn produces even more benzene and xylene isomers. Other common elements of these disproportionation and transalkylation processes include high temperature reaction conditions, consumption and recycle of expensive hydrogen, per-pass conversions significantly below 100%, which leads to large recycles, and energy intensive distillations to recover benzene and C8 aromatics from unconverted feedstocks. There remains a need in the art for improved efficiency in the production of paraxylene by these processes through reducing raw material costs and decreasing the energy consumption.
Disproportionation and transalkylation processes are generally located near paraxylene production facilities in large process plants called aromatic complexes. In addition to disproportionation, transalkylation, and paraxylene units, aromatics complexes also contain facilities for the purification of primarily benzene and toluene via liquid-liquid extraction or extractive distillation. Paraxylene production facilities are generally comprised of one of two technologies. These two technologies are selective adsorption and crystallization. Selective adsorption facilities for the production of paraxylene are much more energy intensive than crystallization facilities. Consequently, the largest energy consumer in aromatics complexes that employ selective adsorption is the selective adsorption unit and little attention has been paid to the energy efficiency of other units in the aromatics complex like the disproportionation and transalkylation units. With the increased interest for energy efficient crystallization processes for the production of paraxylene, there is a need to increase the energy efficiency of disproportionation and transalkylation units because these units are now becoming the largest consumers of energy in the aromatics complex.